Porous membrane of poly(metaphenylene isophthalamide) and process for producing the same

ABSTRACT

The invention provides a porous film having a plurality of connected pores, and a process for its production. The porous film of the invention is a porous film formed from highly heat resistant poly(metaphenylene isophthalamide), and having specific ranges for the open areas and difference between them on both surfaces, as well as specified ranges for the mean pore sizes and porosity on both surfaces. The permeability and impregnation with respect to substances such as air and water, as well as the dynamic strength, are therefore excellent, and the porous film can be used for filters, and for curing resin-impregnated prepregs, multilayer wiring boards, electronic package substrates and the like employing the porous film as a core material.

TECHNICAL FIELD

The present invention relates to a porous film composed ofpoly(metaphenylene isophthalamide) and to a process for its production.The porous film is useful for various precision filters, and formultilayer wiring boards, electronic package substrates and flexibleprinted boards employing the porous film as a core material.

BACKGROUND ART

Polyolefins such as polypropylene have conventionally been known for useas porous films. However, because they have poor heat resistance andundergo dimensional changes due to heat shrinkage of their films andpores when used, for example, in situations where the temperatureexceeds 180° C., problems have occurred such as reduction or loss oftheir function as porous films.

Aromatic polyamides are known as alternative films with excellent heatresistance. Japanese Examined Patent Publication SHO No. 59-14494, forexample, describes a process for production of an aromatic polyamideporous film comprising at least 80 mole percent of a metaphenyleneisophthalamide unit.

Japanese Examined Patent Publication SHO No. 59-36939 describes aprocess for production of a porous film made of an aromatic polyamide.

Both of the aforementioned production processes disclose, for mostcases, addition of an inorganic salt in the dope or coagulation liquid.Consequently, since trace amounts of the inorganic salts remain in thefinal porous film product, they are unsuitable as materials forelectronic uses.

Porous films formed from aromatic polyamides are often subjected toordinary stretching treatment to increase the surface open area, butproblems have often arisen in such cases depending on the use, such asthe problem of increasing mean pore size with stretching treatment.

On the other hand, the rapid development of high-density informationtechnology in recent years has led to an increased demand fordimensional stability, workability, even greater thinness and highdensification, and various types of electronic circuit boards have alsobeen desired therefor. In light of these circumstances, new types ofbase materials employing porous films have been proposed.

Japanese Unexamined Patent Publication No. 2001-345537 and JapaneseUnexamined Patent Publication No. 2002-111227 describe examples ofporous polyamides as multilayer wiring board materials for electronicpackages.

Another use is described, for example, in Japanese Patent PublicationNo. 2,623,331, wherein a porous plastic film made of an aromaticpolyamide is used as a separator for an electrolytic capacitor.

Japanese Unexamined Patent Publication HEI No. 11-250890 discloses theuse of a polyamide film with pores as a porous film for a batteryseparator. Also, International Patent Publication WO01/19906 discloses apoly(metaphenylene isophthalamide) film having a porous structure.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a novel porous filmmade of poly(metaphenylene isophthalamide).

It is another object of the invention to provide a porous film made ofpoly(metaphenylene isophthalamide), wherein the porous film has a highsurface open area.

It is yet another object of the invention to provide a porous film madeof poly(metaphenylene isophthalamide) wherein the porous film has acontrolled surface open area and a highly uniform porosity.

It is still another object of the invention to provide a porous filmhaving the aforementioned characteristics, which is suitable forelectronic circuits or separators.

It is still another object of the invention to provide a process forproduction of a novel porous film made of poly(metaphenyleneisophthalamide) which has a specified surface open area.

Other objects and advantages of the present invention will becomeapparent from the explanation presented below.

According to the invention, these objects and advantages are achieved,firstly, by:

[1] A porous film possessing at least two surfaces and comprising aplurality of connected pores, wherein the porous film

(1) consists substantially of poly(metaphenylene isophthalamide),

(2) has an open area of 20-70% on both of two surfaces of the porousfilm,

(3) has a difference 0-40% in the open areas of two surfaces,

(4) has a mean pore size of 0.1-10 μm on both of two surfaces, and

(5) has a porosity of 30-90%.

The objects and advantages of the invention are achieved, secondly, by:

[2] A process for production of a porous film which is a productionprocess for a porous film possessing at least two surfaces andcomprising a plurality of connected pores, wherein a polymer solutioncontaining poly(metaphenylene isophthalamide) and an amide solvent issubjected to the following steps (i) to (iv) in order:

(i) a casting step of casting onto a support,

(ii) a dipping coagulation step wherein the cast solution layer isdipped in an amide coagulation liquid containing a substance which isnon-compatible with poly(metaphenylene isophthalamide) for coagulationof the cast solution layer,

(iii) a washing and releasing step wherein the coagulated layer obtainedin the previous step is washed and released, or released while washing,from the support, and

(iv) a heat treatment step wherein the washed and released coagulatedlayer film is heat treated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative diagram of an electronic package substrate.

FIG. 2 is an example of an SEM photograph showing the cross-section of aporous film of the invention as seen from the top, the bottom anddiagonally.

FIG. 3 is a cross-sectional view of an SEM photograph for a comparativeexample.

EXPLANATION OF SYMBOLS

1: IC chip

2: Resin

3: Bump

4: Package substrate

5: Terminal

BEST MODE FOR CARRYING OUT THE INVENTION

A first embodiment of the porous film of the invention will now beexplained.

The porous film of the invention is in an ordinary film or sheet formand possesses at least two surfaces. As shown in FIG. 2, for example, aplurality of pores are present in both surfaces and the pores areconnected to form a connected porous structure.

The poly(metaphenylene isophthalamide) of which the porous film of theinvention is composed is, substantially, a polymer obtained bypolycondensation wherein a meta-aromatic diamine and a meta-aromaticdicarboxylic acid halide are reacted in substantially equimolar amounts.It may also be a copolymerized polymer wherein the meta-aromatic diamineis replaced with another amine component such as a para-aromaticdiamine, aliphatic diamine or alicyclic diamine in an amount of nogreater than 20 mole percent with respect to the amount of themeta-aromatic diamine used. Also included are copolymerized polymerscomprising alternative dicarboxylic acid components such as apara-aromatic dichloride, aliphatic dicarboxylic acid or alicyclicdicarboxylic acid in an amount of no greater than 20 mole percent withrespect to the amount of the meta-aromatic dicarboxylic acid halideused. The term “substantially” is used in this sense.

As examples of meta-aromatic diamines there may be mentioned1,3-phenylenediamine, 1,6-naphthalenediamine, 1,7-naphthalenediamine,2,7-naphthalenediamine and 3,4′-biphenyldiamine.

As meta-aromatic dicarboxylic acid halides there may be mentioneddicarboxylic acid dihalides of, for example, isophthalic acid,1,6-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acid and3,4-biphenyldicarboxylic acid.

Among these, using 1,3-phenylenediamine as the meta-aromatic diamine oran isophthalic acid dihalide as the meta-aromatic dicarboxylic acidhalide will provide advantages in terms of properties of the obtainedporous film and cost. Particularly suitable is a combination of1,3-phenylenediamine and an isophthalic acid dihalide.

Specific examples of copolymerizing monomers for the other aminecomponent include para-aromatic diamines such as para-phenylenediamine,4,4′-diaminobiphenyl, 2-methyl-para-phenylenediamine,2-chloro-para-phenylenediamine, 2,6-naphthalenediamine and3,4′-diaminodiphenylether.

Specific examples of copolymerizing monomers for the other dicarboxylicacid component include dicarboxylic acid dihalides such as terephthalicacid chloride, biphenyl-4,4′-dicarboxylic acid chloride,2,6-naphthalenedicarboxylic acid chloride, etc. as para-aromaticdicarboxylic acid dichlorides, aliphatic diamines such as hexanediamine,decanediamine, dodecanediamine, ethylene diamine andhexamethylenediamine, or dicarboxylic acid dihalides such asethylenedicarboxylic acid, hexamethylenedicarboxylic acid or the like asaliphatic dicarboxylic acids. These diamines and dicarboxylic acidhalides may be used alone, or in combinations of two or more.

The poly(metaphenylene isophthalamide) of the invention is preferably apolymer having an inherent viscosity in the range of 0.8-2.5 dl/g andpreferably 1.0-2.2 dl/g, as expressed by the following formula (1):Inherent viscosity (units: dL/g)=ln(T/T ₀)/C   (1)

If the inherent viscosity is lower than 0.8 dl/g it is not possible toachieve sufficient film strength, while if it is higher than 2.5 dl/g itbecomes difficult to obtain a stable polymer solution, and a homogeneousporous film cannot be achieved.

The definitions of T, T₀ and C in formula (1) above are as follows.

-   T: Flow time for a solution of 0.5 g of poly(metaphenylene    isophthalamide) in 100 mL of N-methyl-2-pyrrolidone as measured with    a capillary viscometer at 30° C.-   T₀: Flow time for N-methyl-2-pyrrolidone as measured with a    capillary viscometer at 30° C.-   C: Polymer concentration in polymer solution (g/dL)

The porous film of the invention is characterized by having a surfaceopen area of 20-70% on both the front and back sides. According to theinvention it was found, surprisingly, that specifying a range not onlyfor the size of the pores on both sides of the porous film and theporosity of the porous film, but also for the proportion of surfacepores on both sides of the porous film can result in efficientimpregnation of the resin and water in the porous film.

The porous film of the invention has a surface open area of at least 20%on two surfaces, i.e. both the front and back sides, and thereforeliquids, resins and the like impregnate into the porous film uniformlyin a short time. The limit of 70% is preferred in order to maintain thestrength of the porous film. The surface open area is more preferably20-65% and even more preferably 25-60%. The surface open area can becalculated by image processing of a surface photograph taken by SEMobservation or the like.

The difference between the surface open areas on one side of the porousfilm and the other side is in the range of 0-40%. Generally speaking, asmaller difference will improve the impregnation rate (also referred toas “impregnation efficiency”) mentioned above, and therefore thedifference in the open areas on both sides is preferably 0-20%.

The porous film of the invention has a mean pore size of 0.1-10 μm onboth surfaces. If the mean value is outside of this range, theimpregnation rate (impregnation efficiency) mentioned above will beinsufficient and the strength will be inadequate. The pore size istherefore preferred to be as uniform as possible, as this isparticularly advantageous from the standpoint of impregnation rate,strength and gas permeability. The mean pore size on both sides ispreferably 0.1-3 μm. A size of less than 3 μm is preferred to allowadaptation to fine pitching of wiring when the use is, for example, as acore material for a package substrate.

The porous film of the invention has a porosity of 30-90%. The porosityrepresents the proportion of voids in the porous film, and is calculatedby the following formula (2):Porosity (%)=(1−ρ_(f)/ρ₀)×100   (2)(ρ_(f): apparent density of porous film; ρ₀: true density of polymerused).

Porosity is preferably 50% or greater for expression of the function asa porous film, and preferably no greater than 90% in order to maintainstrength of the porous film. The porosity is more preferably 55-85% andeven more preferably 60-80%.

The gas permeability according to the invention is the value measuredaccording to JIS P8117. The porous film of the invention preferably hasa gas permeability of 0-3600 sec/100 cc. The gas permeability ispreferably not greater than 3600 sec/100 cc for use as a filter, a basematerial for a separator or prepreg, or a core material for anelectronic package substrate, because the connected structure of thepores will be insufficient. The preferred range for the gas permeabilitywill differ depending on the purpose of use and the environment of use,and for example, when the use is as a core material for an electronicpackage substrate, it is preferably in the range of 10-3600 sec/100 ccand more preferably in the range of 10-2000 sec/100 cc.

The porous film of the invention preferably has 0-300 sec/μLpermeability of water through at least one side, and more preferably0-300 sec/μL permeability of water through both sides. Since thepermeability of liquid through the porous film of the invention ispoorer for liquids with high surface tension, limiting the permeabilityfor water, which has a high surface tension, to 300 sec/μL will ensuresatisfactory permeability for other liquids as well. The waterpermeability may be evaluated by dropping 1 μL of a wet tension testliquid with a surface tension of 730 μN/cm according to JIS K6768 ontoboth sides of the porous film allowed to stand for 24 hours or longer inan environment at 23° C., 50% RH, and measuring the time until the testliquid completely permeates into the porous film. The water permeabilityis preferably not greater than 300 sec/μL, because more time will berequired for liquids to evenly impregnate to the interior of the porousfilm, while it will can also be difficult to achieve uniformimpregnation.

A preferred range for the water permeability will differ depending onthe purpose of use and the environment of use, and for example, when theuse is as a base material for a prepreg or as a core material for anelectronic package substrate, the water permeability is preferably inthe range of 1-300 sec/μL and more preferably in the range of 1-200sec/μL.

The porous film of the invention has a gas permeability of no greaterthan 3600 sec/100 cc, a surface open area of 10-70% and a porosity of50-90% on the front and back sides and a water permeability of nogreater than 300 sec/μL on both sides, although these will depend on thepurpose of use.

A process for production of the aforementioned porous film according tothe second invention will now be explained.

The porous film of the invention may be produced by a process forproduction of a porous film which is a production process for a porousfilm possessing at least two surfaces and comprising a plurality ofconnected pores, wherein a polymer solution containingpoly(metaphenylene isophthalamide) and an amide solvent is subjected tothe following steps (i) to (iv) in order:

(i) a spreading step of casting onto a support,

(ii) a dipping coagulation step wherein the cast solution layer isdipped in an amide coagulating solution containing a substance which isnon-compatible with poly(metaphenylene isophthalamide) for coagulationof the cast solution layer,

(iii) a washing and releasing step wherein the coagulated layer obtainedin the previous step is washed and released, or released while washingfrom the support, and

(iv) a heat treatment step wherein the washed and released coagulatedlayer is heat treated.

For the porous film of the invention, the aforementionedpoly(metaphenylene isophthalamide) (A) (hereinafter also referred to as“polymer”) is first dissolved in an amide solvent (B) to prepare apolymer solution (C) (hereinafter referred to as “dope”). Next, the dopeis cast onto a support to produce a cast solution layer on the support.The cast solution layer is then dipped in an amide coagulating solutionfor coagulation of the cast solution layer. Finally, the solidified castsolution layer is released from the support while washing and then heattreated.

<Dope Production Step>

According to the invention, the polymer (A) is dissolved in the amidesolvent (B) which is capable of dissolving the polymer, to prepare adope (C). The temperature for dissolution is not particularly limited solong as it is below the boiling point of the amide solvent (B) used, andit may be, for example, from −20° C. to 200° C.

The concentration of the polymer (A) in the dope (C) is preferably 3-30wt % and more preferably 5-20 wt %. If the dope concentration is outsideof the range of 3-30 wt %, the uniformity of the porous film thicknesswill be impaired and productivity may be reduced.

The amide solvent (B) used may be, for example, a polar amide solventcontaining amide groups, such as N-methyl-2-pyrrolidone,N,N-dimethylacetamide or N,N-dimethylformamide. However, the solvent isnot particularly restricted to these, and any solvent is suitable solong as the object of the invention is not prevented. Techniques foradding inorganic salts to dopes are known in the prior art, as mentionedabove, but since no inorganic salt is used according to the presentinvention, the resulting porous film will contain substantially noinorganic salt. Consequently, the porous film of the invention may beappropriately used for electrical and electronic purposes.

Adding to the dope (C) at least one type of polyhydric alcoholicsubstance (D₁) and/or C5-19 hydrocarbon (D₂) which is soluble in theamide coagulating solution described hereunder is preferred to allowcontrol of the liquid permeability and surface open area of the porousfilm of the invention.

As the polyhydric alcoholic substance (D₁) there is preferably used onewhich has at least two hydroxyl groups in the molecule and dissolves to1 wt % or greater in the coagulating solution at a temperature of 30° C.As examples of specifically preferred compounds there may be mentioneddiol compounds such as ethylene glycol, diethylene glycol and propyleneglycol, polymers such as polyethylene glycol, polypropylene glycol andpolyhydroxyalkyl (meth)acrylate and their copolymers, glycerin and itsderivatives, and the like. Among these, polyethylene glycol andpolyhydroxyalkyl methacrylate are particularly preferred in terms ofeasier control of the surface open area of the obtained porous film. Ascompounds having at least two hydroxyl groups in the molecule there maybe mentioned polyvinyl alcohol and ethylenevinyl alcohol copolymer, butthese compounds are not preferred because they are virtually insolublein amide-based coagulating solutions at 30° C. and thus leave largeresidue of the polymer in the porous film, thus lowering the heatresistance of the porous film.

The content of the polyhydric alcoholic substance (D₁) in the dope (C)is preferably no greater than 100 parts by weight, more preferably nogreater than 50 parts by weight, even more preferably no greater than 20parts by weight and especially no greater than 10 parts by weight to 100parts by weight of the polymer. This is because a polyhydric alcoholicsubstance (D₁) content of greater than 100 parts by weight will tend toleave residue of the polyhydric alcoholic substance (D₁) in theresulting porous film, thereby lowering the heat resistance of theporous film. The polyhydric alcoholic substance (D₁) is added for thepurpose of controlling the physical properties of the porous film, andtherefore its addition will not always be necessary, depending on thepurpose of use. Consequently, while there is no particular restrictionon the lower limit of its content, it will normally be at least 0.01part by weight to 100 parts by weight of the polyhydric alcoholicsubstance (D₁).

The C5-19 hydrocarbon (D₂) is an aliphatic hydrocarbon or aromatichydrocarbon. From the standpoint of easier control of the surface openarea of the obtained porous film, it is preferred to use an aliphatichydrocarbon. From the standpoint of compound stability and economy, asaturated hydrocarbon is preferably used. Examples of specific preferredcompounds include cyclohexane, decane, dodecane, tetradecane,hexadecane, octadecane and liquid paraffin.

These compounds may be used alone or in any combinations of two or more.The C5-19 hydrocarbon may also contain a small amount of a C20 orgreater compound, in which case the latter must constitute no more than40 wt % of the total hydrocarbon. At greater than 40 wt %, the surfacecontacting the coagulating solution tends to be rough, making itdifficult to obtain a homogeneous porous film. The proportion is morepreferably no greater than 20 wt %.

The content of the hydrocarbon (D₂) in the dope (C) is preferably0.01-10 wt %. An amount of less than 0.01 wt % will lower the open areaof the surface of the porous film in contact with the support base,while an amount of greater than 10 wt % will lower the open area of thesurface of the porous film in contact with the coagulateing solution,thus making it impossible to achieve the object of the invention. Therange is more preferably 0.1-8 wt % and even more preferably 0.3-5 wt %.

(i) <Casting Step>

In the production process of the invention, the dope (C) is subsequentlycast onto a support. As examples of supports there may be mentionedglass panels, steel belts, drums or polymer films of polypropylene,polyethylene terephthalate or the like. From the standpoint ofproductivity, a polymer film is preferably used. Such polymer films mayalso be subjected to release treatment with silicon, etc. or coronadischarge treatment, or the like.

It was found, surprisingly, that subjecting the support to rubbingtreatment is effective for accomplishing the object of the invention.Rubbing treatment involves rubbing in one direction with a cloth or thelike, and such rubbing treatment allows control of the open area of thesurface of the porous film in contact with the support surface.

The rubbing pressure and number of times for rubbing may beappropriately selected as the conditions for rubbing treatment. Thepreferred range for the rubbing pressure is 10-1000 g/cm², with 100-800g/cm² being more preferred. A rubbing pressure of less than 10 g/cm² mayresult in an insufficient rubbing effect. A rubbing pressure of greaterthan 1000 g/cm² may increase damage to the support, while also hasteningwear of the rubbing cloth itself. There is no particular restriction onthe number of times for rubbing, and it may be appropriately selected toobtain the desired open area for the porous film.

The temperature of the dope (C) cast onto the support is notparticularly restricted. However, the dope viscosity is important interms of film formation and because it affects the properties of theresulting porous film. The viscosity is preferably selected between1-2000 Poise and more preferably selected between 5-500 Poise. In orderto maintain a sheet-like form of the cast solution layer, it iseffective when carrying out the invention to select the atmospherictemperature of the support and its surroundings, and to adjust theatmosphere surrounding the support by blasting or the like. Theatmospheric temperature will depend on the type of polymer used, thedope viscosity and the dope concentration, but in most cases will be inthe range of 5-50° C.

(ii) Dipping Coagulation Step

The cast solution layer is then promptly dipped into an amidecoagulating solution for coagulation of the cast solution layer.

Examples of specific amide solvents to be used for the amide coagulatingsolution include N-methyl-2-pyrrolidone, N,N-dimethylacetamide andN,N-dimethylformamide, which may also be used in combinations of two ormore. From the viewpoint of control of the porous structure,N-methyl-2-pyrrolidone is preferably used at 100% as the amide solvent.

The amide coagulating solution contains a compound which issubstantially incompatible with the polymer (A). Such a compound is onewhich is inert with respect to the polymer (A) and amide solvent, andwhich is basically compatible with the amide solvent. As examples ofsuch compounds there may be mentioned water, lower alcohols, lowerethers and the like, which may also be used in combinations of two ormore. Using water alone is particularly advantageous from the standpointof film properties of the obtained porous film, and economy.

The concentration of the amide solvent (for example, 100%N-methyl-2-pyrrolidone) in the amide coagulating solution, in the caseof a mixed solution with water, for example, is 50-80wt %, morepreferably 50-70 wt % and even more preferably 55-70 wt % with respectto the total coagulating solution. If the amide solvent concentration isless than 50 wt %, the open area of the porous film surface will tend tobe reduced, and the gas permeability and water permeability will tend tobe lower. If the concentration is greater than 80 wt %, more time willbe required to obtain an independent porous film, and the productivitywill therefore be undesirable.

The temperature of the amide coagulating solution is preferably arelatively low temperature, with a lower limit of, for example, −20° C.,preferably −10° C. and more preferably 0° C. The upper limit ispreferably 80° C. and more preferably 60° C. Depending on the use,however, this limit may be set to no higher than +25° C., and morepreferably between −10° C. and +20 C. Formation of the film with acoagulateing solution temperature of between −20° C. and +25° C. willyield a polymer porous film with satisfactory water permeability.

If the temperature is below −20° C. with an amide solvent concentrationof less than 50 wt %, the number of pores in the produced polyamideporous film surface will be reduced, while the pore sizes will also tendto be smaller, tending to yield a polyamide porous film with a low openarea. If the concentration is greater than 80% and the temperatureexceeds 80° C., the pore sizes will tend to be larger and it will oftenbe impossible to obtain a porous film of the invention. If either of thetemperature or concentration exceeds the ranges specified above,disadvantages may result depending on the use, although not as much aswhen both are outside of the specified ranges.

The time for dipping of the cast solution layer in such an amidecoagulating solution is not particularly restricted but will normally befrom 10 seconds to 60 minutes. If the dipping time is too short theinternal structure of the porous film becomes inhomogeneous, and it ispreferably not too long from the standpoint of productivity.

In order to confer higher temperature heat resistance to the porous filmof the invention, the obtained coagulated layer is preferablycrystallized after the dipping coagulation step. The crystallizationmethod is not particularly restricted, but a method of dipping in theaforementioned amide coagulating solution is preferred from thestandpoint of productivity.

When such a crystallization step is employed, the concentration of theamide solvent in the dipping treatment bath is preferably 50-80 wt % andmore preferably 60-70 wt %. The temperature is preferably 40-98° C. andmore preferably 50-90° C. If the concentration of the amide solvent inthe dipping treatment bath is greater than 80 wt %, the polyamide porousfilm will sometimes dissolve leading to breakdown of the porousstructure, while a concentration of less than 50 wt % may result ininsufficient crystallization. A dipping treatment bath temperature ofbelow 40° C. will either prevent or hamper crystallization of thepolyamide porous film, while a temperature of above 98° C. isundesirable as it will tend to cause dissolution of the polyamide porousfilm and breakdown of the porous structure. The dipping treatment timemay be appropriately determined in consideration of the resulting filmproperties and productivity, but in order to achieve the object of theinvention, the treatment time is preferably selected so that the heat offusion of the porous film after heat treatment in the final step is inthe range of 10-80 J/g as measured with a differential scanningcalorimeter (DSC) at 10° C./min. For example, when thepoly(metaphenylene isophthalamide) is produced from meta-phenyleneisophthalamide and isophthalic chloride, dipping treatment for between30 seconds and 60 minutes can adjust the heat of dissolution to withinthe aforementioned ranges.

The cast solution layer dipped in the amide coagulation solution will bedischarged from both surfaces of the cast solution layer by dissolution,etc. of the amide solvent from the surface of the cast solution layer inthe coagulating solution. At the same time, pores are formed byinfiltration of the non-compatible compound in the coagulating solutioninto the cast solution layer. Presumably, the interior of the castsolution layer solidifies almost simultaneously with formation of theporous structure having connected pores. In this case, the sizes andshapes of the pores and the interior porous structure are assumed tochange depending on the polymer concentration, dope concentration, dopecomposition, support material, amide coagulating bath composition andconcentration, dipping time, temperature, etc. These conditions may beappropriately determined according to the desired properties for thefinal porous film product.

The solidified porous cast solution layer is then conveyed to a washingstep where it is washed with water. The washing temperature is notparticularly restricted since there is virtually no effect on the poreshapes.

(iv) Heat Treatment Step

The washed porous cast solution layer is then dried in a heat treatmentstep (drying step). The drying method is not particularly restricted,and may be any method from drying by nip roll treatment for simpledraining, to thorough heat drying with a hot air drier or the like.

The crystallized porous film can become brittle depending on the dryingconditions, and therefore in the washing and releasing step and/or heattreatment step it is important to achieve 5-30% shrinkage in terms ofarea ratio based on the area at release of the porous film from thesupport. The shrinkage is preferably not less than 5% because theobtained porous film may be extremely brittle and difficult to handle,possibly leading to breakage of the porous film in the heat treatmentstep after the drying step, while the shrinkage is also preferably notgreater than 30% because shrinkage irregularities may be produced,making it impossible to obtain a homogeneous porous film. A morepreferred range for the shrinkage is 10-25%.

The porous film of the invention obtained in this manner may besubjected to an additional final stage of heat treatment in order toconfer dimensional stability against heat. The heat treatment conditionsfor an amorphous porous film preferably include a temperature of200-300° C. Heat treatment at below 200° C. is not preferred because theeffect of improved dimensional stability will be reduced, and heattreatment at above 300° C. is not preferred because the glass transitiontemperature of the polymer will be exceeded, resulting in breakdown ofthe porous structure. A more preferred temperature range is 240-280° C.The conditions for heat treatment of a crystalline porous filmpreferably include a temperature of 200-380° C. Heat treatment at below200° C. is not preferred because the effect of improved dimensionalstability will sometimes be reduced, and heat treatment at above 380° C.is not preferred because decomposition of the polymer may occur. A morepreferred temperature range is 240-340° C. The heat treatment time maybe appropriately determined in consideration of the obtained filmproperties and productivity and is not particularly restricted, but itmay be selected so that the heat of fusion is in the range of 10-80 J/gas measured with a differential scanning calorimeter (DSC) at 10°C./min, as mentioned above. The treatment time will normally be about5-60 minutes.

The crystalline porous film produced in this manner preferably has aheat shrinkage of 0-0.7% when treated at 260° C. for 10 minutes. Thisrepresents dimensional stability at high temperature, and can beachieved by the range for heat of fusion described above. The heatshrinkage when treated at 260° C. for 10 minutes is preferably low, andit is more preferably no greater than 0.6% and even more preferably nogreater than 0.5%.

EFFECTS OF THE INVENTION

According to the invention, there is provided a porous film havingexcellent dynamic strength and heat resistance, as well as satisfactorysubstance permeability. Since the porous film is easily impregnated withvarious liquids, it can be used, for example, as a prepreg byimpregnation of a curing resin such as an epoxy resin. It is also usefulas a core material for multilayer wiring boards, electronic packagesubstrates and the like, and for various precision filters. Needless tomention, two or more porous films according to the invention may also belaminated for use. FIG. 1 shows a conceptual diagram for a method of useas a core material for an electronic package substrate.

EXAMPLES

The present invention will now be explained through the followingexamples, with the understanding that the invention is not limited inany way to these examples. The measurement methods for the porous filmswere as follows.

(1) Surface Open Area

A surface photograph at 2000× magnification observed with a scanningelectron microscope with a resolving power of 4-7 nm was developed to150 mm vertical×200 mm horizontal, and a scanner was used at aresolution of 100,000 pixels/30,000 mm², and the number of pixels foreach pore with a diameter of 0.01 μm or greater was calculated,recording the total as the number of pixels of the open portion. Thesurface open area was determined by the following formula.Surface open area=Total pore pixels/100,000 pixels×100 (%)

(2) Mean Pore Size

A surface photograph at 2000× magnification observed with a scanningelectron microscope with a resolving power of 4-7 nm was developed to150 mm vertical×200 mm horizontal, and a scanner was used at aresolution of 100,000 pixels/30,000 mm², the number of pixels for eachpore with a diameter of 0.01 μm or greater was calculated, and the totalwas divided by the number of pores to determine the mean pore area, fromwhich the diameter was calculated for a perfect circle.

(3) Porosity

The dried porous film was cut to a size of A (mm)×B (mm), and thethickness C (mm) and weight D (g) were measured (with appropriateselection of A, B, C and D). The apparent density E was then determinedby the formula shown below. The true density F of the polymer was alsodetermined and the porosity was calculated by the formula shown below.Apparent density E=D/(A×B×C)×1000 (g/cm³)Porosity=(F−E)/F×100 (%)

(4) Gas Permeability

The time for permeation of 100 cc of air at a pressure of 0.879 g/mm²was determined according to JIS P8117, and expressed as the Gurleynumber.

(5) Tensile Test

The test was conducted at a pull speed of 10 mm/min in an atmosphere of23° C., 50% RH according to JIS K7110, and the tensile strength,breaking elongation and Young's modulus were measured.

(6) Heat Shrinkage

The heat shrinkage was determined by treatment at 260° C. for 10minutes, according to JIS K7133.

(7) Inherent Viscosity (IV)

A 0.5 g portion of the polymer was dissolved in 100 mL ofN-methyl-2-pyrrolidone, and a capillary viscometer was used to determinethe inherent viscosity at 30° C.Inherent viscosity (units: dL/g)=ln(T/T ₀)/C   (1)

-   T: Flow time for the polymer solution as measured with a capillary    viscometer at 30° C.-   T₀: Flow time for N-methyl-2-pyrrolidone as measured with a    capillary viscometer at 30° C.-   C: Polymer concentration in the polymer solution (g/dL)

(8) Water Permeability

After dropping 1 μL of a wet tension test liquid with a surface tensionof 730 μN/cm according to JIS K6768 onto the front and back sides of theporous film which had been allowed to stand for 24 hours or longer in anenvironment at 23° C., 50% RH, the time until the test liquid completelypermeated into the porous film was measured.

(9) Evaluation of Impregnation

Approximately 5 μL of an epoxy resin solution was dropped onto thesurface of the porous film at room temperature, and the condition ofimpregnation into the porous film was visually examined and evaluated onthe following scale. The epoxy resin used was “DER” 736 by Dow Corp.

∘: Uniform impregnation immediately after dropping

Δ: Non-uniform but gradual impregnation

×: Virtually no impregnation

Poly(metaphenylene isophthalamide)(CONEX by Teijin Techno Products Co.,Ltd.) was used as the polymer in all of the examples, and its inherentviscosity (IV) was 1.4 with measurement at a polymer concentration of0.5 g/dL and a temperature of 30° C., with NMP (N-methyl-2-pyrrolidone)as the solvent. When sulfuric acid was used as the solvent andmeasurement was conducted under the same conditions, the IV was 1.8.This will hereinafter be referred to as “Conex polymer”.

Example 1

A porous film was produced under the conditions shown in Table 1, in thefollowing manner.

The Conex polymer was dissolved in N-methyl-2-pyrrolidone and thepoly(metaphenylene isophthalamide) concentration was adjusted to 10 wt%. The dope was cast onto a polypropylene film (rubbed 30 times with acontact pressure of 140 g/cm²) to a thickness of 140 μm. The castsolution layer was then introduced for 5 minutes into a 15° C.coagulating bath comprising 60 wt % N-methyl-2-pyrrolidone and 40 wt %water to obtain a coagulated layer. The coagulated layer was releasedfrom the polypropylene film and dipped into a 50° C. water bath for 30minutes. The coagulated layer was then treated at 120° C. for 30 minutesand subsequently at a temperature of 270° C. for 30 minutes to obtain apoly(metaphenylene isophthalamide) porous film.

The properties of the porous film are shown in Table 1 and indicate arelatively high surface open area and satisfactory gas permeability. Thetensile strength was 22 MPa, the breaking elongation was 62% and theYoung's modulus was 750 MPa, indicating satisfactory dynamic strength.The heat shrinkage was 0.8%, and therefore the porous film had veryexcellent dimensional stability. The impregnation and permeability forwater were also high. The epoxy resin impregnation of the porous filmwas also excellent. It is therefore expected to have satisfactoryadhesion with copper foil and to be useful for a prepreg.

Example 2

A porous film was produced according to Example 1, under the conditionsshown in Table 1.

The properties of the obtained porous film are shown in Table 1. As inExample 1, the liquid permeability, impregnation and dynamic propertieswere excellent.

The epoxy resin impregnation was also good, indicating satisfactoryadhesion with copper foil, for use as a prepreg.

Example 3

Exactly the same procedure was carried out as in Example 1 and the castsolution layer was introduced into a coagulating bath to prepare acoagulated layer. The coagulated layer was then released from thepolypropylene film, and fixed to a metal frame to prevent shrinkage ofthe coagulated layer. It was then introduced for 30 minutes into a 65°C. coagulating bath comprising 50 wt % N-methyl-2-pyrrolidone and 50 wt% water. The coagulated layer was removed from the metal frame anddipped in a 50° C. water bath for 30 minutes. Upon completion ofdipping, the shrinkage was 9.8% in terms of area ratio. The coagulatedlayer was then dried at 120° C. for 30 minutes. At this point, theshrinkage was 19% in terms of area ratio. Treatment was then carried outat a temperature of 280° C. for 30 minutes to obtain a porous film.

The thickness of the obtained porous film was 55 μm, the porosity was70%, the open area of the front side (the side not contacting thepolypropylene film) was 30%, the open area of the back side (the sidecontacting the polypropylene film) was 37%, the mean pore size on thefront side was 1.0 μm, the mean pore size on the back side was 0.7 μmand the heat of fusion was 32 J/g. The gas permeability was 160 sec/100cc, indicating satisfactory gas permeability, the tensile strength was25 MPa, the breaking elongation was 32% and the Young's modulus was 1050MPa, indicating satisfactory dynamic strength. Also, the heat shrinkageupon treatment at 260° C. for 10 minutes was 0.40%, and therefore theporous film had very excellent dimensional stability.

The porous film had excellent liquid permeability and impregnation. Theepoxy resin impregnation was also good, indicating satisfactory adhesionwith copper foil, for use as a prepreg.

Example 4

The Conex polymer was dissolved in N-methyl-2-pyrrolidone, after whichcyclohexane (special grade, product of Wako Pure Chemical Industries)was added, and the poly(metaphenylene isophthalamide) concentration wasadjusted to 10 wt % and the cyclohexane concentration was adjusted to 2wt %. The dope was cast onto a polypropylene film to a thickness of 140μm. The cast solution layer was then introduced for 5 minutes into a 5°C. coagulating bath comprising 58 wt % N-methyl-2-pyrrolidone and 42 wt% water to obtain a coagulated layer. The coagulated layer was releasedfrom the polypropylene film and dipped into a 50° C. water bath for 30minutes. After completion of the dipping, the coagulated layer wastreated at 120° C. for 30 minutes and subsequently at a temperature of270° C. for 30 minutes to obtain a porous film.

An electron microscope photograph (SEM photograph) of this porous filmis shown in FIG. 2. FIG. 2(a) shows the surface condition on one side ofthe porous film (the side not in contact with the polypropylene film).FIG. 2(b) shows the surface condition on the other side of the porousfilm (the side formed in contact with the polypropylene film). FIG. 2(c)shows the interior structure of the porous film as seen from a slant.

All of the surfaces had pores with approximately uniform pore sizesacross the entire surface. The interior structure was also a uniformporous structure with a plurality of connected pores.

The properties of the obtained porous film are listed in Table 1. As inthe previous examples, the liquid permeability, impregnation and dynamicproperties were excellent.

The epoxy resin impregnation was also good, indicating satisfactoryadhesion with copper foil, for use as a prepreg.

Example 5

The Conex polymer was dissolved in N-methyl-2-pyrrolidone, after whichpolyethylene glycol (weight-average molecular weight: 1000, product ofWako Pure Chemical Industries) was added for adjustment of thepoly(metaphenylene isophthalamide) concentration to 10 wt % and thepolyethylene glycol concentration to 0.5 wt %, to prepare a dope. Thedope was cast onto a polypropylene film to a thickness of 100 μm, andthen introduced for 10 minutes into a 0° C. coagulating bath comprising60 wt % N-methyl-2-pyrrolidone and 40 wt % water. The coagulated layerwas then released from the polypropylene film and dipped into a 30° C.water bath for 30 minutes to obtain a coagulated layer. The coagulatedlayer was treated at 120° C. for 30 minutes and subsequently at atemperature of 270° C. for 30 minutes to obtain a porous film.

The properties of the obtained porous film are listed in Table 1.Specifically, the porous film had a thickness of 42 μm, a porosity of66%, an open area on the front side (the side not contacting thepolypropylene film) of 25%, an open area on the back side (the sidecontacting the polypropylene film) of 43%, a mean pore size on the frontside of 1.1 μm and a mean pore size on the back side of 1.0 μm. The gaspermeability was 266 sec/100 cc, indicating satisfactory gas.permeability. The tensile strength was 28 MPa, the breaking elongationwas 53% and the Young's modulus was 850 MPa, indicating satisfactorydynamic strength. Also, the water permeability was 80 sec/μL on thefront side and 75 sec/μL on the back side, and therefore the porous filmhad excellent liquid permeability.

The epoxy resin impregnation was also good, indicating satisfactoryadhesion with copper foil, for use as a prepreg.

Example 6

A porous film was produced according to Example 5, under the conditionsshown in Table 1. (Weight-average molecular weight: 2 million, productof Wako Pure Chemical Industries.)

The properties of the obtained porous film are shown in Table 1. As inExample 5, the liquid permeability, impregnation and dynamic propertieswere excellent.

The epoxy resin impregnation was also good, indicating satisfactoryadhesion with copper foil, for use as a prepreg.

Example 7

A porous film was produced according to Example 5, under the conditionsshown in Table 1.

The properties of the obtained porous film are shown in Table 1. As inExample 5, the liquid permeability, impregnation and dynamic propertieswere excellent. (PHEMA: poly(2-hydroxyethyl methacrylate)(Viscosity-average molecular weight: 300,000, product of Aldrich.)

The epoxy resin impregnation was also good, indicating satisfactoryadhesion with copper foil, for use as a prepreg.

Example 8

A porous film was produced according to Example 5, under the conditionsshown in Table 1.

The properties of the obtained porous film are shown in Table 1. As inExample 5, the liquid permeability, impregnation and dynamic propertieswere excellent. (EG: ethylene glycol)

The epoxy resin impregnation was also good, indicating satisfactoryadhesion with copper foil, for use as a prepreg. TABLE 1 Comp. Comp. Ex.Ex. Example 1 2 3 4 5 6 7 8 1 2 Film Dope conc. wt % 10 8 10 10 10 10 1010 8 10 forming Dope viscosity poise 220 120 220 215 220 220 220 220 120220 condi- (30° C.) tions Additive — — — cyclo- PEG PEG 2 PHEMA EG — —hexane 1000 million Addition wt % — — — 2 0.5 0.05 0.1 2 — — Support PPPP PP PP PP PP PP PP PP PP rubbing rubbing rubbing Coating μm 140 60 140100 100 100 100 100 100 200 thickness Coagulateing NMP % 60 55 60 58 6060 60 60 40 55 solution ° C. 15 15 15 5 0 20 20 0 30 30 min 5 5 5 5 10 55 10 5 10 Crystallization NMP % — — 50 — — — — — — — ° C. — — 65 — — — —— — — min — — 30 — — — — — — — Water bath ° C. 50 50 50 50 30 30 30 3050 50 min 30 30 30 30 30 30 30 30 30 30 Heat treatment 1 ° C. 120 120120 120 120 120 120 120 120 130 min 30 30 30 30 30 30 20 30 30 30 Heattreatment 2 ° C. 270 270 280 270 270 270 270 — — — min 30 30 30 30 30 3030 — — — Film Film thickness μm 50 15 55 39 42 45 50 41 38 60 evalua-Porosity % 76 63 70 70 66 66 72 68 83 70 tion Front open area % 30 63 3051 25 26 24 30 9 25 Back open area % 37 40 37 38 43 27 25 38 10 10 Frontpore μm 1.4 1 1 0.8 1.1 1 1.2 1.6 1 1.6 diameter Back pore μm 0.8 0.50.7 1.2 1 0.7 0.7 0.9 0.4 0.7 diameter Permeability sec/100 130 15 160145 266 300 250 180 42 100 (Gurley value) cc Tensile Mpa 22 32 25 25 2827 25 27 6.1 — strength Breaking % 62 55 32 45 53 55 60 55 6.4 —elongation Young's modulus Mpa 750 1300 1050 880 850 890 820 860 234 —Heat shrinkage % 0.8 0.8 0.4 0.85 — — — — — — DSC J/g — — 32 — — — — — —— Front water sec/μL 143 120 165 176 80 72 120 60 240 155 permeabilityBack water sec/μL 95 135 148 160 75 68 150 60 253 350 permeability Epoxyresin ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Δ Δ impregnationFront: Side produced not in contact with polypropylene filmBack: Side produced in contact with polypropylene film

Comparative Example 1

A porous film was produced under the conditions shown in Table 1. Thespecific procedure was as follows.

The Conex polymer was dissolved in N-methyl-2-pyrrolidone and thepoly(metaphenylene isophthalamide) concentration was adjusted to 8 wt %.The dope was cast onto a polypropylene film to a thickness of 100 μm.The cast solution layer was then introduced for 5 minutes into a 30° C.coagulating bath comprising 60 wt % N-methyl-2-pyrrolidone and 40 wt %water to produce a coagulated layer. The coagulated layer was releasedfrom the polypropylene film and dipped into a 50° C. water bath for 30minutes. The coagulated layer was then treated at 120° C. for 30 minutesto obtain a porous film. The SEM photograph of a cross-section of theporous film (FIG. 3) shows that the interior structure of the porousfilm was extremely non-uniform.

The properties of the porous film are listed in Table 1, and theyindicate a high porosity. However, the surface open area was extremelylow and the gas permeability was also low, indicating a satisfactory gastransmission rate. The breaking elongation and Young's modulus weresmall, indicating weak dynamic strength. The epoxy resin impregnationwas also inadequate.

Comparative Example 2

A porous film was produced under the conditions shown in Table 1. Thespecific procedure was as follows.

The Conex polymer was dissolved in N-methyl-2-pyrrolidone and thepoly(metaphenylene isophthalamide) concentration was adjusted to 10 wt%. The dope was cast onto a polypropylene film to a thickness of 200 μm.The cast solution layer was then introduced for 10 minutes into a 10° C.coagulating bath comprising 55 wt % N-methyl-2-pyrrolidone and 45 wt %water to produce a coagulated layer. The coagulated layer was releasedfrom the polypropylene film and dipped into a 50° C. water bath for 30minutes. The coagulated layer was then treated at 130° C. for 30 minutesto obtain a porous film.

The properties of the porous film are listed in Table 1 and indicate ahigh porosity. However, the surface open area was considerably low onthe back side (the side in contact with the polypropylene film), thewater permeability was low, and the epoxy resin impregnation was alsoinadequate.

INDUSTRIAL APPLICABILITY

According to the present invention, the porous film comprisingpoly(metaphenylene isophthalamide) and having a plurality of connectedstructures in its interior is characterized by having a specific openarea, a difference in open areas on both sides within a specified rangeand a mean pore size and porosity within specific ranges on bothsurfaces, thereby exhibiting excellent permeability and impregnation forsubstances such as air and water and excellent dynamic strength. Theporous film is therefore useful for filters, and for curingresin-impregnated prepregs, multilayer wiring boards, electronic packagesubstrates and the like employing it as a core material.

1. A porous film possessing at least two surfaces and containing aplurality of connected pores, wherein the porous film (1) consistsessentially of poly(metaphenylene isophthalamide), (2) has an open areaof 20-70% on both of two surfaces of the porous film, (3) has adifference 0-40% in the open areas of two surfaces, (4) has a mean poresize of 0.1-10 μm on both of two surfaces, and (5) has a porosity of30-90%.
 2. A porous film according to claim 1, wherein a waterpermeability is 0-300 sec/μL for penetration from at least one surface.3. A porous film according to claim 1, wherein the difference in theopen areas of two surfaces is 0-20%.
 4. A porous film according to claim1, wherein a heat of fusion is 10-80 J/g as measured by DSC at 10°C./min.
 5. A porous film according to claim 4, wherein a heat shrinkageis 0-0.7% upon treatment at 260° C. for 10 minutes.
 6. A porous filmaccording to claim 1, which has a thickness of 5-100 μm.
 7. A porousfilm according to claim 1, which contains substantially no inorganicsalt.
 8. A porous film according to claim 1, wherein a value of a gaspermeability measured according to JIS P8117 is 0-3600 sec/100 cc.
 9. Aprocess for producing a porous film which is a production process for aporous film possessing at least two surfaces and containing a pluralityof connected pores, wherein a polymer solution containingpoly(metaphenylene isophthalamide) and an amide solvent is subjected tothe following steps (i) to (iv) in order: (i) a casting step of castingonto a support, (ii) a dipping coagulation step wherein the castsolution layer is dipped in an amide coagulating solution containing asubstance which is non-compatible with poly(metaphenyleneisophthalamide) for coagulation of the cast solution layer, (iii) awashing and releasing step wherein the coagulated layer obtained in theprevious step is washed and released, or released while washing from thesupport, and (iv) a heat treatment step wherein the washed and releasedcoagulated layer is heat treated.
 10. A process for production of aporous film according to claim 9, wherein the support surface issubjected to rubbing treatment before the polymer solution is cast ontothe support.
 11. A process for production of a porous film according toclaim 10, wherein the pressure for rubbing treatment which is appliedonto the support is 10-1000 g/cm².
 12. A process for production of aporous film according to claim 9, wherein step (ii) is followed by astep of dipping in the amide coagulating solution with the cast solutionlayer in a coagulated state and then heat treatment, for crystallizationof the coagulated layer.
 13. A process for production of a porous filmaccording to claim 12, wherein the coagulated layer is caused to shrink5-30% in terms of area ratio in the crystallization step.
 14. A processfor production of a porous film according to claim 9, wherein thepolymer solution also contains, as additives, a polyhydric alcoholsubstance and/or a C5-19 hydrocarbon which is soluble in the amidecoagulating solution.
 15. A process for production of a porous filmaccording to claim 9, wherein the amide coagulating solution comprisesN-methyl-2-pyrrolidone and water which is incompatible withpoly(metaphenylene isophthalamide), and the N-methyl-2-pyrrolidoneconstitutes 50-80 wt % of the total amide coagulating solution.
 16. Aprocess for production of a porous film according to claim 9, whichinvolves at least one action selected from the group consisting ofrubbing treatment according to claim 10, use of additives according toclaim 14 and a crystallization step according to claim
 12. 17. Anelectronic package substrate comprising a porous film according to claim1 as the core material.
 18. Use of a porous film according to claim 1 asthe core material for an electronic package substrate.